1. Field of the Invention
This invention relates to a method for producing large amounts of redox shuttle and more specifically, this invention relates to a method for producing at least one kilogram (kg) per batch of a phosphate-containing redox shuttle having a first oxidation potential of approximately 5 volts (V).
2. Background of the Invention
Lithium-ion battery advances are the de facto power sources for portable electronic devices and vehicle propulsion. Rechargeable lithium-ion batteries are used for thousands of cycles with no gaseous exhaust, delivering high density energy and providing reliable and clean chemical energy storage. Compared to fossil fuels and biomass, lithium-ion batteries compare favorably in reducing air pollution and therefore global warming.
As lithium battery chemistry increases in voltage potential, so too does the need to prevent overcharging. One means for preventing overcharging is the use of electronic monitoring devices attached to each cell to monitor voltages.
Another means is the use of anti-overcharge additives to electrolyte. These additives include redox shuttle molecules. Generally, redox shuttle molecules can be reversibly oxidized and reduced at a defined potential slightly higher than the end-of-charge potential of the cathode. This mechanism can protect the cell from overcharge by locking the potential of the cathode at the oxidation potential of the shuttle molecules. Redox shuttles have been implemented for overcharge protection of 3 V class lithium-ion batteries.
Efforts have been made to provide redox shuttles with overcharge protections in excess of 4 volts (V). However, state of the art shuttle production protocols result in extremely small yields (e.g., less than 10 grams). Also, those methods require inert environments and expensive reagents such as chlorodiethyl-phosphite. Reaction 1 depicts a two-step protocol for producing the redox shuttle 2,5-di-tert-butyl-1,4-phenylene tetraethyl bis(phosphate).
Reaction 1:

Another drawback to the state of the art synthesis is that it is a multi-step process. Specifically, the heretofore utilized synthesis of RS-6 was conducted in two steps. Diethylchlorophosphite was reacted with 2,5-di-tert-butylbenzene-1,4-diol in the first step, and then the intermediate phosphite compound was treated with tert-butyl hydroperoxide in the second reaction to produce the phosphate compound RS-6 with a yield of approximately 70 percent. This two-step process uses a large amount of dichloromethane. Dichloromethane is also toxic, carcinogenic and presents disposal and environmental dangers.
State of the art shuttle production protocols utilize large amounts of solvent. Large solvent volumes are not only expensive, but also present environmental and safety issues. Also, oxidation typically requires hydroperoxide, which is hazardous on a large scale. Also, the procedure requires anhydrous solvent that needs to be handled under an inert atmosphere and protected from moisture.
A revised protocol of the synthesis of this shuttle uses diisopropyl-ethylamine and diethylchlorophosphite at −78° C., and oxidized with tert-butyl hydroperoxide. However, this protocol still has scale up issues with the need for cryogenic conditions, peroxides, and halogenated solvent. Particularly, the cryogenic conditions are very energy intensive and require specialized equipment.
A need exists in the art for a method for producing redox shuttles for use in batteries that yields at multiples of 500 gram quantities per processing cycle. The method should not require elaborate processing parameters nor should the method require expensive and dangerous reagents. Also, the processing cycles should last no more than 5 to 7 hours, and conducted in ambient environments wherein no controlled atmosphere is required.